This invention concerns grinding and polishing machines and methods of grinding and polishing discs such as wafers of silicon for use in the construction of semi-conductor devices, and discs of glass or other brittle materials on which magnetic material is to be deposited for forming magnetic memory disks for computer disk drives and the like.
When grinding discs for either of the above purposes, it is important that the outside diameter of the disc is finished to a high level of accuracy and often to a particular cross-sectional form. In the case of memory disk a circular opening is also required again to an accurately controlled diameter and circularity. In the case of silicon wafers, registration in later manufacturing steps requires registration devices to be formed around the periphery of the disc such as the formation of flats and notches.
Conventionally edge grinding and polishing machines have incorporated linear slideways for all axes. Whether incorporating re-circulating rolling element bearings or air bearings, all such axes share a common failing when grinding and polishing brittle material namely they permit significant relative motion between grinding wheel and component. This arises from the need to provide for orthogonal movements of grinding wheels usually obtained by stacking one linear axis above another. This motion requires the use of hard wearing grinding wheels to minimise loss of form due to wear, but such wheels tend to produce poor quality surfaces with deep damage.
When grinding silicon wafers, the depth of sub-surface damage arising during grinding should be minimised as a wafer must be delivered with zero damage for use in subsequent manufacturing steps. When sub-surface damage has arisen, this means a post grinding step of acid etching before polishing. Both processes are expensive, and the less the damage the shorter the polishing time that is needed.
Normally the edge of a wafer or disc workpiece is machined so as to create a generally triangular or trapezoidal edge profile to the disc. In order to achieve this the surface of the grinding wheel is formed in a complementary manner so that as the wheel is engaged with the workpiece the complementary form is generated around the edge thereof.
The edge profile will only be generated accurately if the complementary form of the grinding wheel does not alter. Clearly as the grinding wheel wears, this profile will change and periodically such wheels have to be re-formed using a suitable forming wheel for resin bond wheels or spark erosion for metal bond wheels. Hitherto at least the initial forming of the complementary grinding wheel profile has been done xe2x80x9coff machinexe2x80x9d since hard wearing grinding wheels have tended to be used.
In accordance with one aspect of the present invention a grinding machine includes at least one forming wheel which is mounted so as to be capable of being engaged with the main grinding wheel to allow a groove in the latter to be formed and reformed as required.
This obviates problems which can arise when a grinding wheel is initially formed off machine and/or has to be removed to be reformed.
By constructing a grinding machine as hereinafter described, so as to restrict, if not eliminate, unwanted relative motion between workpiece and grinding wheel, it has been found that softer grinding wheels can be used.
Resin bonded CBN or diamond wheels may be employed, although optimum efficiency may be obtained by using a harder grinding method such as a metal bonded CBN or diamond wheel in a preliminary rough grinding step and then using the softer wheels to ginish grind. The different wheels can of course all be mounted on the same spindle.
Each grinding wheel may be provided with a plurality of grooves, which can be used in turn, as each in turn becomes worn, until all the grooves need re-forming.
Resin bonded wheels have been found to produce very low sub-surface damage when used to grind edge forms, and can be formed in situ (ie in position on the grinding machine) using a suitable forming wheel.
Preferably and in accordance with another aspect of the invention, the forming wheel may be mounted on the workhead spindle, typically to the rear of the workpiece support.
Two forming wheels may be mounted on the workhead, one for forming grooves in new wheels and one for reforming grooves on existing wheels. Forming and reforming grinding wheel grooves on the machine and mounting the forming wheels on the workhead spindle reduces grinding wheel run-out to a minimum, reduces wheel chatter, and also ensures that even if imperfectly mounted, a true circular disc or wafer will be ground by the grinding wheel since the forming wheel, and reforming wheels are both mounted about the same axis as the workpiece.
Using one forming wheel for roughing and one for finishing both grinding wheels produces an identical form in both the roughing and finishing wheel grooves, thereby minimising asymetric wear of the finishing wheel form.
Using slightly different forms in the two forming wheels ensures that a constant depth of material is removed from around the edge profile during finish grinding.
Mounting two completely different forms, allow rapid changeover between one form type and the other, without the need to change formers.
The use of a resin bonded CBN wheel for rough grinding wafers and discs as aforesaid represents an important advantage. If a coarse diamond grain grinding wheel were to be used it would be virtually impossible to form or reform the wheel using diamond formers.
Coarse and medium grain CBN is very easy to form using a diamond forming wheel. However fine grain CBN such as required for finish grinding wheels will not grind silicon. Finish grinding is therefore preferably achieved using a fine grain diamond wheel which can be formed and reformed using diamond forming wheels.
The invention thus also lies in the combination of a tight stiffness loop between grinding wheel and workpiece support and the use of a relatively compliant resin bonded grinding wheel, together with a forming wheel which may be mounted on the wheelhead spindle, and which can be brought into play as and when required. These features, when combined, allow a high surface finish and low sub-surface damage edge to be produced on a workpiece.
The invention additionally provides a method of grinding the edges of discs comprising the steps of providing a grinding wheel and a forming wheel on a stiff mounting, moving the grinding wheel and forming wheel into engagement so as to form a groove around the edge of the grinding wheel corresponding in cross-section to the complement of the shape to be formed around a disc edge during grinding, engaging the grinding wheel with a disc which is also stiffly mounted relative to the grinding wheel, grinding the edge thereof into the desired shape, and after the disc edge has been ground (or after a succession of disc edges have been ground), re-engaging the grinding wheel and the forming wheel to re-form the groove in the grinding wheel to correct for wear.
A grinding process for grinding the edge of a silicon wafer workpiece may comprise two stages, a first in which a metal or resin bonded CBN wheel is advanced to rough grind the workpiece edge and a second stage in which a resin bonded CBN, or more preferably a resin bonded diamond wheel, is advanced to finish grind the edge, although the invention is not limited to this process and any number of different grinding steps may be incorporated into the process set up to grind any particular workpiece.
In accordance with the invention the method also includes the steps of forming and reforming both grinding wheels in situ on the machine by plunge grinding the desired form in the edge of each grinding wheel using a forming wheel.
Where both internal and external diameters are to be finish ground, a two-stage grinding process may be employed for both internal and external grinding, in which a rough grind using a pre-formed metal bonded wheel precedes a finish grind using a formed CBN resin bonded wheel which is formed and reformed in situ on the machine.
The invention also lies in apparatus for performing the above method comprising a grinding wheel and drive means therefor, a forming wheel, support means for the grinding wheel and forming wheel which is stiff to restrict unwanted relative movement therebetween, means for axially shifting the grinding wheel into alignment with the forming wheel, means for effecting relative movement between grinding and forming wheels to plunge grind the edge of the grinding wheel to form a groove therein of precise cross-section, a workpiece spindle on which a circular workpiece can be mounted, said spindle also being carried by the stiff support means, drive means for moving the grinding wheel towards and away from the workpiece spindle, to bring the wheel into engagement with a circular edge of a circular workpiece when mounted on the spindle to edge grind the workpiece, and drive means for rotating the spindle wherein the forming wheel is also mounted on the workpiece spindle for rotation therewith.
Preferably the workpiece spindle includes a vacuum chuck for mounting a circular workpiece thereon.
Optical inspection means may be provided for viewing the face of the workpiece when mounted for rotation, so that a plurality of measurements can be made and compared with reference measurements.
Other aspects and features of the invention are defined in the appended claims.